The Fairfax County Board of Supervisors has approved a zoning change for George Washington’s historic River Farm that would require additional oversight before the property could be developed.

The move on Tuesday — which builds on earlier measures passed by the county and the Virginia General Assembly to preserve River Farm — comes as a former Trump administration member has put forward an ambitious plan to buy the site, along with two adjoining parcels on the Potomac River, and build a $300 million luxury resort.

A. Wayne Johnson, a former Education Department official who ran unsuccessfully for the U.S. Senate in last year’s special election in Georgia, proposed purchasing the land for $20 million from its current owners, the American Horticultural Society (AHS), and preserving its 27 acres as green space with public access. Johnson also offered to invest an additional $10 million or more to restore River Farm’s gardens and renovate its manor. But he said the evenly divided AHS board has so far decided not to pursue his idea.

“We were going to — or we would, it still may be possible — but I sense it may be more difficult than originally envisioned,” Johnson said in an interview.

River Farm’s proposed sale triggered efforts at all levels of government to preserve a parcel that once formed part of George Washington’s Mount Vernon estate, and it plunged the nonprofit’s leadership into a bitter internal struggle.

Half of AHS’s current board members have argued that River Farm had become too costly to maintain and hindered the nonprofit’s plans to expand its educational programs on a national scale. The other half have said the venerable gardening society purchased River Farm with a $1 million gift from philanthropist Enid Annenberg Haupt nearly 50 years ago and should honor her express wish that it remain as AHS headquarters and a green space open to the public.

Attorneys general in Virginia and the District, where AHS is chartered, have both opened investigations into the legality of the nonprofit’s proposed sale of a charitable asset.

Johnson’s offer to purchase River Farm had increased anxiety among neighbors, public officials and preservationists who mobilized last year after AHS announced plans to sell the property on the open market.

Alan Rowsome, executive director of Northern Virginia Conservation Trust, was one of the first to sound the alarm about Johnson’s proposal. Rowsome, whose organization is part of a coalition seeking to purchase or establish a partnership with AHS to preserve River Farm, issued a statement last week even as he acknowledged that he lacked key details about the offer, including the buyer’s identity.

“The reality is sometimes these things are happening in the back rooms and the fix ends up in — and I can’t risk that, after everything we’ve put into it, knowing that there’s very good evidence of this happening,” Rowsome said at the time.

AHS said Tuesday in a written statement that the organization remained focused on negotiations with the Northern Virginia Regional Park Authority (NOVA Parks) and had hoped the county might postpone the Board of Supervisors’ vote while discussions were underway.

“The American Horticultural Society (AHS) is not involved with and cannot comment on any of the neighborhood properties surrounding River Farm,” the organization’s statement says. “The only offer the AHS board is currently entertaining is an offer from NOVA Parks, and we are pleased with the ongoing positive spirit of these negotiations.”

Five members of the organization’s evenly split board — Skipp Calvert, Tim Conlon, Laura Dowling, Holly Shimizu and Marcia Zech — issued a statement through attorney John T. Richards, Jr. saying they opposed any sale of the land.

“As we have stated before, we … oppose the sale of River Farm. Period,” they said. “We believe that Enid Haupt’s restrictive gift is very clear — that her intent was that River Farm should be owned and operated by AHS as its national headquarters while making the property available for the public to enjoy.”

During Tuesday’s board meeting, Supervisor Daniel G. Storck (D-Mount Vernon) urged the AHS board to work with a mediator to bridge their differences.

“We still end up with a deadlocked board, and the deadlocked board means we really don’t have finality to the next steps of what River Farm is going to be,” Storck said in an interview.

Johnson, who ran a private student loan company and built residential communities for seniors before then-Education Secretary Betsy DeVos appointed him to oversee the department’s financial aid operations, described himself as the principal behind River Farm Heritage LLC and its proposal to build “a world class” resort on about 54 acres. He said he envisioned River Farm with a new permanent headquarters for AHS, a “boutique culinary school” and a Michelin-starred restaurant set amid newly restored gardens above the Potomac River.

Two adjoining properties immediately to River Farm’s south — for which Johnson said he has entered letters of intent to purchase — would become the main revenue-generating center of the complex, including a clubhouse suitable for weddings and similar events, and additional housing that would help sustain River Farm. Johnson said these properties, too, would have more public access than they do now as private residences.

Johnson, who lives in Macon, Ga., but has kept an apartment in Washington, said his plan has additional financial backing from investors in Latin America. And while AHS has so far decided not to pursue his idea, Johnson said he would be open to further discussions.

“We would certainly be willing to pick the conversation back up,” Johnson said. “My suspicion is that the horticultural society has got to reconcile itself with what it really wants to do.”

The zoning measure passed Tuesday by the Board of Supervisors may make that more difficult. It was initiated following the Virginia General Assembly’s passage of a measure sponsored by Sen. Scott A. Surovell (D-Fairfax) that was designed solely to protect River Farm. The new state law empowers Fairfax County — which in April created a historic overlay district to preserve River Farm’s heritage — to write additional zoning provisions to regulate development and guarantee public access.

Under the new zoning amendment, the Architectural Review Board must review and approve any development plans for River Farm — also known as Wellington — to ensure that these are compatible with its historic nature.

The zoning amendment also empowers the Board of Supervisors to further impose any condition it deems necessary because of its historical significance, including requiring public access.

The Board of Supervisors’ vote Tuesday was 9 to 0, with an abstention by Supervisor Pat Herrity (R-Springfield). He has expressed concerns that the manner in which the county altered the zoning had unfairly encroached on property rights.